An attempt by one of the Gaza-bound “freedom flotilla” ships to defy the Greek government and escape from port was thwarted on Monday when armed coastguard officials caught up with the vessel and forced it back to shore.

On a day that activists had dubbed “make or break” for the international coalition of boats seeking to break Israel’s blockade of Gaza, the Canadian ship Tahrir burst out of Agios Nikolaos port in Crete at 6pm local time after supporters blocked the coastguard with manned kayaks.

“We have left port [and] are full steam ahead – coastguard boat about 5-10 [minutes] behind us,” announced passengers on the ship’s official Twitter feed as they raced towards international waters. But the faster coastguard boat caught up with the Tahrir and prevented it from going any further.

“Our boat has just been illegally boarded by armed members of the Greek coastguard and commandeered against our will,” Dylan Penner, a member of Tahrir steering committee, told the Guardian by phone from the ship’s deck. “This is conclusive evidence that Israel’s unlawful siege on Gaza has now been extended to Greece.”

The captain of a US ship, The Audacity of Hope, was arrested after a similar failed attempt to flee the port in Athens last week.

The Greek government caused controversy on Friday when it banned all flotilla ships from leaving its ports, without explaining its reasons. Critics accused the beleaguered government of bowing to Israeli and US pressure and surrendering Greek sovereignty over its sea borders. The Israeli prime minister, Binyamin Netanyahu, thanked his Greek counterpart for helping to stamp out “anti-Israeli provocation”.

The flotilla’s last hope now appears to lie with the Stefano Chiarini, a Dutch-Italian vessel currently docked on the island of Corfu. Campaigners on the boat say they are expecting to get legal clearance to sail on Tuesday and believe they have local political support for their mission, a claim seemingly confirmed on Monday by the local governor.

Spiros Spirou, the provincial official in charge of the Ionian islands, told the Guardian that he “admires and supports the activists’ struggle” and said he would make no attempt to stop the ship if it left harbour. “Greece loves peace, but at this moment it can’t confront more powerful economic forces,” claimed Spirou, adding that official attempts to tie the flotilla up in bureaucracy and paperwork were merely a pretext to preventing it from sailing at all.

“The ban has come from the ministries in Athens and I have no responsibility for it at all – I’ve tried to get in contact with them and get an explanation but I have not been able to get through,” he said. “Right now Greece is in crisis and decisions have been taken at an international level.”

Elsewhere, flotilla activists vowed not abandon their mission, despite the growing number of seemingly irreversible setbacks. “Everyone involved with this flotilla came with the determined intention to break Israel’s illegal siege on Gaza,” said Penner. “This is not over yet.”